A multiplicity of local coils including one or more receiving antennas for magnetic resonance signals may be used in magnetic resonance imaging apparatuses. Owing to the dictates of the structural design and in particular for cost reasons, it is possible that an assigned receiving unit may not simultaneously acquire the signals of all the receiving antennas. It is possible, in particular, that the same receiving antenna configuration is intended to be usable with different receiving units having a different number of channels.
The limiting factors when guiding reception signals from the receiving antenna to a processing unit in this case include the number of transmission links, e.g., the number of signal lines present, and the receiving possibilities of the receiving unit, such as the number of analog-to-digital conversion channels present or the number of further processing stages of the receiving unit.
It is known to use so-called mode matrices in order to superpose the reception signals of a plurality of receiving antennas in a complexly weighted fashion, e.g., with a respective amplitude and a respective phase offset. In this case, the signals of a plurality of receiving antennas are superposed such that a so-called primary or CP mode in the center of the examination region has a particularly high signal-to-noise ratio. In addition, higher modes are formed, (for example, the so-called secondary mode and/or tertiary mode), which map the edge regions of the examination region particularly well. If, by way of example, a single receiving channel is intended to be used for the group of receiving antennas fed into such a mode matrix, then exclusively the primary mode may be acquired. In this case, the reception signals may be superposed in the mode matrix in such a way that the number of output signals provided is exactly the same as the number of reception signals fed in, wherein the reception signals may be completely reconstructed during the evaluation of all the output signals.
In order to reduce the number of required transmission channels, it is known, moreover, to combine the different modes of a mode matrix to form a common transmission signal by the use of a frequency division multiplexer. What is disadvantageous in this case is that a separate transmission link and a separate input at the receiving unit have to be provided for each of the mode matrices used.